The medical community, companies involved in the development and formulation of photoprotective consumer products, the popular press, and regulators have recognized the long term health consequences of exposure to ultraviolet radiation. It is well known that exposure to UVB light (290 nm-320 nm) causes erythema and sunburn. Longer term, exposure to short and long wavelength UVA rays (from about 320 nm to 400 nm) has been linked not only to photodamage—manifested as fines lines, rhtyids, lentigines, uneven pigmentation—but also actinic keratoses, impaired immune function and skin cancers.
Organic sunscreen filters prevent erythema and photodamage by absorbing a certain percentage of UVR over a specified spectrum. Sun protection factor (“SPF”) is an expression of this percentage and theoretically indicates that the user is protected X times longer than without sunscreen. An SPF 33 product would, theoretically, allow 3% of unattenuated sunlight to reach the skin. Such a product would absorb 97% of erythemal UVR, allowing the user to conclude that he or she could stay out in the sun 33 times longer than without the sunscreen without erythema.
More recently, there has been increased focus on the contribution of UVA to these adverse health effects. This increased awareness of the importance of UVA protection resulted in the introduction of broad-spectrum sunscreens (having protection across the ultraviolet radiation spectrum, from 290 nm to 400 nm) and in FDA's Proposed Amendment of Final Sunscreen Monograph published in the Federal Register in Vol. 72, No. 165 at pages 49070 to 49122.
FDA is now proposing that both in vitro and in vivo tests be conducted to determine UVA radiation protection. The proposed in vitro test is the ratio of long wavelength UVA-1 absorbance (340 nm-400 nm) to total UV absorbance (i.e., UVB+UVA). The proposed in vivo test relates to persistent pigment darkening (PPD) test, which is similar to the SPF test except the endpoint is pigment darkening rather than erythema. FDA is proposing that UVA labeling consist of a UVA rating reflecting both the in vitro and in vivo test results, with sunscreen formulations that offer the highest UVA protection receiving a maximum of four stars.
Achieving broad-spectrum protection across the ultraviolet radiation spectrum (from 290 nm to 400 nm) requires sunscreen active ingredients that absorb, block or otherwise attenuate longer UVA-1 radiation. Avobenzone, zinc oxide and titanium dioxide are three sunscreen actives currently approved by the FDA that provide protection from longer UV wavelengths. (The recently approved Mexoryl SX sunscreen is effective against shorter wavelengths, having a maximum absorption at 344 nm.)
It is well-known in the art, however, that avobenzone rapidly undergoes photochemical degradation and is, therefore, not considered to be “photostable”. Thus, the labeled SPF of a sunscreen formulation containing avobenzone is not necessarily reflective of the product's UVR protection.
Exposure to UVR dissipates the available concentration of the organic UV filters in a sunscreen formulation, resulting in a product with decreased efficacy, both in terms of level of protection and time of protection. In terms of photophysics, UVR exposure causes organic UV absorbing molecules to leave the ground state and enter the single-excited state. The excited-state filters may be returned to the ground state by singlet quenching (and thereby conserved to continue providing UVR protection), or transferred from the singlet-state to the triplet-excited state. In the triplet state, the UV filter molecule may undergo triplet quenching (and return to the ground state). If, however, triplet quenching (or another photophysical pathway) does not restore the sunscreen molecule to the ground state, the filter undergoes photodegradation by one or more photochemical reactions. Photodegraded filters lose their ability to filter UVR, thereby decreasing the effectiveness of the overall formulation.
In part recognizing the limitations of SPF, other metrics for sunscreen efficiency have been developed. Among these is photostability. In published articles and meetings of national and international Societies of Cosmetic Chemists, researchers have commented that broad-spectrum protection must remain efficient throughout the period of UVR exposure.
As used in the present application, the photostability of a UV filter can be measured and expressed in terms of the amount (i.e., concentration) by which the filter is degraded by one or more photochemical reactions, including photofragmentation, isomerization, tautomerization, photoaddition and substitution reactions, and/or cycloadditions. These photodegradation reactions also generate free radicals, which are associated with adverse health consequences.
Previous attempts to create photostable sunscreen formulations containing avobenzone are described in U.S. Pat. Nos. 7,235,587; 7,244,416; 6,444,195; 6,033,649; 6,071,501; 5,985,251; 5,849,273; 5,576,354; and 5,587,150. (To the extent pertinent, all published US patent application and granted US patents cited in the present application are incorporated by reference in their entirety.)
U.S. Pat. No. 5,776,439 teaches a photostable composition comprising from 1% to 10% avobenzone and from 0.5% to 10% oxybenzone, a UVB absorber. (Unless otherwise stated, percentages are weight/weight.)
Oxybenzone has a triplet excited state energy greater than 65 kcal/mol. The triplet excited state energy of avobenzone is less than 60 kcal/mol. At the September 2007 Annual Sunscreen Symposium organized by the Florida Chapter of the Society of Cosmetic Chemists, Bonda described several photophysical pathways for quenching singlet excited energy, including fluorescence. (Fluorescence is the emission of photons from the singlet excited.) Bonda reported that oxybenzone quenches avobenzone fluorescence and proposed a mechanistic explanation for the increased photostability of avobenzone in formulations containing oxybenzone. According to Bonda, oxybenzone likely reduces the flow of singlet excited state energy to the triplet-excited state, and thereby decreases the potential for photodegradative chemical reactions involving avobenzone.
Published U.S. Patent Application No. 2005/0013781 teaches a sunscreen composition comprising homosalate, octyl salicylate, oxybenzone, octocrylene and avobenzone.
There remains a need for a broad-spectrum sunscreen formulation having a high UVA rating of PFA +++ that exhibits a high degree of photostability and further comprises an effective antioxidant system to quench free radicals generated by photodegradation of sunscreen filters. This need is met by the composition of the present invention.